As exemplified in FIG. 1A, in optical signal transmission using an optical fiber 1 serving as a transmission path, optical propagation velocity is different depending on its wavelength; therefore, distortion of a signal pulse increases in proportion to transmission distance. In FIG. 1A, when an optical pulse P1 on the transmission side is transmitted through the optical fiber 1, the optical pulse P1 becomes an optical pulse P2 as a result of pulse distortion generated by wavelength dispersion of the optical fiber 1.
This phenomenon is called wavelength dispersion, and has a deep impact on the quality of optical signals in recent optical communication systems which have remarkably higher bit rates. Single mode fiber (hereinafter, referred to as SMF) generally used for the optical communication system at present has a wavelength dispersion of approximately 15 to 17 ps/nm·km near a wavelength of 1550 nm. That is, the optical pulse transmitted for 100 km using the SMF receives a dispersion of approximately 1500 ps/nm, and a difference is generated in reach time for each wavelength component within a signal pulse band.
Wavelength dispersion compensation (hereinafter, referred to as dispersion compensation) is to regenerate the original signal waveform form the optical signals distorted by the influence of the wavelength dispersion, by giving wavelength dispersion of inverse sign against a transmission path to the distorted optical signal. For example, in a dispersion compensator 2 illustrated in FIG. 1B, an optical pulse P2′ which has almost the same waveform to the original signal waveform (P1) is regenerate by giving the wavelength dispersion inverse to the transmission path 1 to the distorted optical pulse P2. At present, the one used most commonly in the present dispersion compensation is dispersion compensating fiber (hereinafter, referred to as DCF).
The DCF is a fiber designed to have dispersion inverse to an ordinary SMF by a special refractive index distribution. In the case of performing long distance optical transmission, a relay node is provided each at a predetermined distance, the DCF is connected thereto, and the DCF is used so that the total amount of dispersion becomes nearly zero.
On the other hand, a super high speed transmission system such as those at 40 Gbit/sec or 100 Gbit/sec has been introduced in order to meet rapidly increasing in communication demands in recent years. In such high bit rate transmission system, dispersion tolerance has been further reduced than that in conventional systems because of its wide signal band; therefore, it is considered that fluctuation in temperature of wavelength dispersion which has not been a problem conventionally needs to be compensated.
Consequently, in such a high speed transmission system of 40 G or more, in addition to a fixed DCF, a variable dispersion compensator which can arbitrarily control the amount of dispersion compensation is considered to be essential in order to compensate for a residual dispersion component which cannot be compensated by the DCF. Hitherto, various kinds of variable dispersion compensators have been proposed.
In the variable dispersion compensators, it is desired to improve device performances in every aspect, such as improvement in accuracy of dispersion compensation and suppression of insertion loss as well as reduction in device size.    (Patent Document 1) Japanese Patent Application Laid-Open No. 2003-264505    (Patent Document 2) Japanese Patent Application Laid-Open No. 2007-298968    (Patent Document 3) Japanese Patent Application Laid-Open No. 2006-221075